1. Field of the Invention
The present invention generally relates to computer media libraries, and more particularly to a library access system with workload balancing and a method for improving the performance of media libraries by balancing the workload between accessors.
2. Description of Related Art
Library access systems are used to house large quantities of data. The data are stored on media such as magnetic tapes, optical or magnetic discs, or other suitable long-term storage.
Conventional library access systems use multiple accessors, essentially dedicated robots, to move media between storage locations and mounting locations, often known as "drives" where the media can be read from and/or written to. These accessors are typically mounted on tracks alongside the media storage locations and are controlled by computer algorithms developed for positional control and indexing of the media. Specific portions of the library can be accessed by looking up the specific media containing desired data, moving an accessor to the location, using the accessor to move the media to the mounting location, and mounting the media in a drive. When the use of the media is complete, the accessor retracts the media and returns it to its storage cell.
Library access systems organize the cells in frames, which may have one or more drives located in them. The frames typically contain hundreds of storage cells and the access systems may contain tens of frames.
Various control algorithms have been developed for optimizing the operation of library access systems, and various schemes for the relationship of accessors to drives have been devised. In some systems, the accessors are assigned to a specific, fixed zone in which one or more drives are located. This has a disadvantage in that when all of the drives in the zone are busy, data located in the zone is inaccessible.
More sophisticated techniques have been used, in which accessors are allowed access to the full library, but either the entire control of the library is integrated in one control system to avoid collisions or the accessors inter-communicate for this purpose. The library is typically divided into zones, where a particular zone is accessed by one accessor, but when that accessor is busy or the drives in that zone are busy, another accessor or another drive can be used to accomplish the retrieval and mount of the media.
A primary access zone is the zone primarily assigned to an accessor. Under low use conditions, an accessor will just operate in its primary zone. But, as system use increases, accessors will increasingly need access to drives or media outside of their primary access zone. Systems using the above techniques must provide a means for avoiding collisions. The typical collision avoiding means is to move another accessor out of the way of an accessor that needs access to a drive or media.
The above techniques improve the performance of the library, but they do not solve a particular problem: some zones of the library may see more use than other zones, and this affects the potential throughput of the system. If all drives can be used equally, this will maximize the use of the system and will also make maintenance more uniform while simultaneously increasing reliability.
Umegaki (JP Kokai 4-313854) describes a method for balancing "shifting mechanism" workloads to increase reliability, and accomplishes this by shutting down one of the accessors when the use of one accessor is greater than the use of a second accessor by a threshold. This will increase reliability, but does not improve performance over the single accessor design, since only the accessor with a lower action count will be operated.
It would, therefore, be desirable to provide a method for balancing the workloads of accessors in library access systems in order to improve performance. This would also tend to equalize the mechanical wear of the accessors, so as to provide faster throughput and increased reliability, but allowing two or more accessors to operate at the same time.